Complete cab solutions
reduced winter heating load in even many northern states to near southern
states levels.

You see them at
the side of roads, in parking lots and pull-off areas of highways—Class 8 over-the-road trucks with their diesel engines idling through the night to run
the heating or air conditioning that assure comfort in the resting drivers' sleeper compartments. A lot of fuel might be saved—and CO2 emissions reduced—with energy-efficient alternatives to extended idling.

The National
Renewal Energy Laboratory, a federal facility based in Golden, CO, took on the
challenge. The idle fuel-usage
studies it found were from 2005; the outdated figures
estimated idling is 7% of total diesel fuel consumption, so NREL focused on
individual Class 8 trucks, proposing a package of technologies that can annually save 774 gallons on A/C cooling load
alone compared with an idling truck; at $3 per gallon for diesel fuel, full payback of an
investment in sleeper cabin climate-control equipment would come in about three years.

Only about half
the states in the U.S. currently restrict idling for over-the-road trucks and laws often are observed in the breach, as some
answers raise cost and reliability issues.

Driver comfort important

The trucking industry and regulators know there's a clear opportunity to save fuel
and improve air quality. But resting
drivers can't be confined to cocoon-like compartments with an occasional wisp
of cool air in summer and/or wrapped in heavy blankets in winter.

Today's sleeper
compartments make driver comfort and convenience high priorities. Volvo VNL Class 8 series single-bed units are
118 ft³/3.34 m³ with a flat roof, and
two-driver models with high roofs (77 in/1955 mm) have bunk beds in a 371 ft³/10.5 m³ living
space—considerable area to be
climate-controlled.

NREL's name
points to its research in such areas as photovoltaics, wind and biosciences,
but much of its work is on cost-effective solutions to real-world
"lower-tech" problems. It has
provided considerable study on climate-control related electric-vehicle range issues
and at the 2016 SAE World Congress explained its extensive research into reducing Class 8
truck idling fuel consumption.

There are alternative approaches for climate control in
sleeper cabs to avoid extensive idling, including
battery-powered auxiliary A/C, fuel-fired
or electric heaters and small diesel engines that serve as auxiliary power units (APUs). The popularity of diesel APUs peaked some
eight years ago and the adoption rate today represents just 9%, explained
Jason Lustbader, NREL senior research engineer.

NREL looked at
many aspects of what it proposes as "complete cab solutions,"
with emphasis on low-cost additions or substitutions. "The project goal was to reduce cab
thermal loads to enable smaller, lighter and more cost-effective idle-off climate-control equipment," explained
Lustbader. The goal: a 30% reduction in
big-rig sleeper climate-control loads with no-idle solutions that pay back
within three years or less.

The NREL project
evaluated four aspects in an integrated strategy: the solar envelope (as overall A/C energy use is in the
equation), heat and cold conductive pathways into the sleeper, efficient
equipment and the volume of the sleeper. In this effort, NREL works with industry partners, particularly Volvo
Group NA, PPG and Aearo Technologies, which makes energy/sound absorbing
materials.

35.7% reduction in A/C load

NREL testing
showed its complete cab solution exceeded the goal, with its most advanced
approach producing a 35.7% reduction in A/C load and 43% decline in heating load, with
an even greater reduction of 53% with a more advanced approach to
insulation. Translating those savings into dollars, NREL found that with the complete cab solution to
reduce loads and adding a battery-powered A/C system for the sleeper, 774 gallons of fuel per year could be saved. Savings
and cost analysis for heating the cabin is still underway, although some
preliminary data was announced.

Prior testing by
NREL indicated the best bang for the buck was in specific insulation, interior curtains, window shades and paints. Although all the actual testing was done at
NREL headquarters, the data was plugged into NREL's own load
estimating software, CoolCalc, to give the results a nationwide scope. The process provides fuel-use estimation by
combining thermal loads with an A/C performance model to calculate an
electrical demand. The load was combined
with modeling to be able to determine fuel use from recharging the battery pack. Because long-haul trucks operate across the
country, the model used the 200 most representative weather stations nationwide
to calculate total fuel saved.

Details that led to NREL's quantified results:

Thinsulate in "plus" package

Baseline
insulation consists of foam attached to the door and body trim panels of the
sleeper. The advanced package instead
uses one- and two-inch insulation blankets with a thermal conductivity rating of
0.03-0.05 W/m-K. In addition to that
advanced package is a "plus" addition of 0.25-in (6.35-mm) layer of 3M's Thinsulate, a synthetic fiber thermal insulation material which has a reflective radiant barrier. It is installed between the interior trim
and the structure of the sleeper.

The three levels
of insulation for the sleeper were combined with three different packages of
privacy shades for the cab glass and sleeper curtains between the cab and
sleeper: standard insulation with
advanced shades and curtains, advanced insulation with standard shades and
curtains and a package of fully advanced insulation, shades and curtains. The first two packages produced almost the
same reduction in UA (heating load): 20.6% and 20.7%. The fully advanced setup resulted in a 43%
reduction.

"Advanced
plus" insulation also was evaluated against standard shades and curtains, advanced
shades and open curtains and advanced shades and curtains. This
"Complete Cab Plus" configuration yielded the greatest reduction
in UA at 53%.

However, even
with Plus insulation and advanced shades, leaving the sleeper curtains open to avoid a
claustrophobic-feeling sleeper caused the
heating load reduction to drop markedly to just 21.6%.

No-idle equipment

A Dometic no-idle
7000-BTU A/C system was installed. This truck sleeper system is battery-operated, has a three-speed blower and will run for more than
11 hours, the company claims. The system was powered by the laboratory's 120-v A/C for
testing to obtain the most accurate
data. However, the analysis assumed the use of
104 A-h AGM (absorbent glass mat) batteries that would recharge while the
truck is in operation.

For heating,
NREL chose a forced-air heater with the diffuser
oriented to avoid air stratification. The heater was operated at 90°F (32°C) to provide a sufficient gap compared with
ambient temperate for accurate measurement of the heat transfer coefficient (clearly much
higher than would be used solely for sleeper comfort). A fuel-fired heater will be evaluated in
future testing and it is expected to enhance the overall results.

Paint-color effect

Exterior paint color affects sleeper thermal loads only when the sun is out for a
long time and intensity is high, typically in summer. However, because it does affect the total vehicle A/C load, the chosen white color was found to deliver a 20.8% load reduction compared with black. As an alternative, solar-reflective blue was
compared with conventional blue and showed a 7.3% saving.

NREL's complete
solution delivered results that equate to operating the trucks in year-round moderate temperatures. Modeling with CoolCalc showed major
reductions in the number of both heating and cooling days in all parts of the
country. This seems to provide an opportunity to save by downsizing the battery
pack and other components in a fully optimized Class 8 climate-control system.

In the ongoing design and engineering crusade that is vehicle lightweighting, grams count. In some cases, tenths of a gram count. Regardless of how fuel price has moderated in the U.S. in recent years, the importance of global platforms and global markets means the drive to create more mass-efficient vehicle structures, subassemblies and components continues unabated.